Recombinant protein expression systems facilitate the production of protein, polypeptides and peptides for screening in drug discovery and for a wide range of applications including vaccine production. Bacterial expression systems have been the main method of choice, largely due to the ease of cloning genes in bacteria, although yeast and baculovirus provide reliable alternative expression systems.
Despite the wide use of recombinant expression systems for the production of proteins, available methods cannot be relied upon to produce any given protein in sufficient yields and having sufficient homogeneity to meet downstream requirements. Protein yield can be a major problem associated with the over-expression of proteins in bacteria, where the protein is directly toxic for the host cell, or where its accumulation disrupts host cell metabolism or growth. Large proteins, as well as proteins with a complex secondary and tertiary structure, tend to be insoluble and accumulate in inclusion bodies in the host cell, where they are generally better tolerated by the host. However, smaller proteins tend to be soluble and their accumulation may be toxic. A number of vectors systems are designed to express the target recombinant protein as a fusion protein with a short or longer N-terminal peptide tag. Such tags, such as the histidine-, or maltose binding-tag are particularly useful for the subsequent purification of the recombinant proteins. There remains however a need for an efficient expression system, especially for therapeutic proteins that are potentially toxic and difficult to express.